Theresa May arriving at the BBC to appear on The Andrew Marr show on Sunday.
Photograph: Mark Thomas/REX/Shutterstock

Theresa May faces a concerted rebellion from the hard Brexit wing of the Conservative party on Monday, as MPs unhappy with her Chequers compromise prepare to mount a show of strength by voting for their amendments on the customs bill.

The party’s European Research Group says it will reject any last attempts at compromise by Number 10 as they hope to force May to change course over Brexit or risk a no-confidence vote before the summer break by demonstrating the depth of their support.

A special ERG whipping operation, using the WhatsApp messaging service, has been created by Steve Baker, the former Brexit minister who resigned from the government last week, although ERG insiders would not put a number on how many they expected to rebel in the Commons.

Jacob Rees-Mogg, the chairman of the ERG, told the BBC “we’ll have an idea of the numbers at 10pm on Monday evening” while one ERG insider added that they were “intensely relaxed” about the number of rebels they had signed up.

The level of support they attract will draw intense focus, particularly if the number significantly exceeds the 48 required to call for a vote of no confidence in May’s leadership of the Conservative party.

May’s leadership has been precarious since she unveiled her proposed new Brexit negotiating strategy at her Chequers residence and detailed it in a white paper last week. It prompted the resignation of Boris Johnson and David Davis from the cabinet, as well as the departures of Baker and a string of more junior ministers.

Q&A

Cabinet: who's in and who's out

WHO'S IN:

Jeremy Hunt

The longest-serving health secretary - having held the role for almost six years - becomes foreign secretary after Boris Johnson's resignation.

Matt Hancock

The former culture secretary replaces Hunt as health secretary.

Jeremy Wright

The former attorney general takes on Hancock’s old job as culture secretary.

Geoffrey Cox

The Devon MP is promoted to be attorney general and will attend cabinet.

Dominic Raab

The former housing minister is promoted to Brexit secretary after the resignation of David Davis.

WHO'S OUT:

David Davis

Goes to the backbenches after kicking off the ministerial reshuffle with his resignation as Brexit secretary over Theresa May's negotiation plans.

Boris Johnson

The gaffe-prone and controversial foreign secretary joins Davis on the backbenches after also quitting his role.

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On Sunday, Robert Courts, the MP who who took over David Cameron’s old constituency said he was resigning as a parliamentary private secretary to the Foreign Office so that he could vote against the government in the customs bill. He said that he had taken the “very difficult decision” to resign “to express discontent” in votes. He added: “I cannot tell the people of WOxon [West Oxfordshire] that I support the proposals in their current form.”

Johnson returned to the political fray with his first column for Monday’s Daily Telegraph since he resigned from the government over the Chequers compromise. It was dedicated to “global Britain,” the slogan or theme of his two years at the Foreign Office. The former cabinet minister said he would “resist – for now – the temptation to bang on about Brexit” and did not discuss the circumstances or reasons for his resignation or relationship with May.

Defending his record in office he added “don’t tell me that the UK is losing diplomatic influence” citing the international response to the Novichok poisoining which saw 28 countries expel 153 “Russian spooks”.

Reports at the weekend suggested that about 40 of the 48 MPs needed had lodged no-confidence letters with the chairman of the party’s backbench 1922 Committee after May had indicated that the UK was prepared to sign up to a “common rule book” for food and goods after Brexit.

There is a belief in Conservative circles that a no-confidence vote in May will have to be called by Wednesday this week as there will otherwise not be time to hold it before parliament breaks up for the summer on Tuesday 24 July. To win, the prime minister has to win over half of the Tory party’s 316 MPs, and if she does, another no-confidence vote could not be held for 12 months.

Speaking to the BBC’s Andrew Marr, May indicated she would seek to fight off any challenge that emerged before the summer recess, saying: “I want to focus people’s minds on how you ensure you achieve that prize, the benefits of leaving the European Union.” She added: “I have always said I’m in this for the long term.”

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Theresa May says Trump told her to 'sue EU' – video

The prime minister also said that the taxation (cross border trade) bill, which seeks to allow the UK to implement its own customs regime after Brexit, and a related trade bill on Tuesday, were “an important part of our no-deal preparations” because both were necessary to prepare the UK for life as an independent trading nation.

The most talked-about of the four amendments proposes to make it illegal for HMRC to collect duties for the European Union without reciprocal arrangements, after May proposed that the UK would collect tariffs on goods heading for the EU after Brexit. It has been signed by 11 Tory MPs including former leader Iain Duncan Smith, former cabinet minister Priti Patel and Rees-Mogg.

The report stage of the bill will also see MPs debate a long-awaited amendment that would keep the UK in the EU customs union post-Brexit that is supported by Labour, the SNP, the Lib Dems and two Tory Remainers, Anna Soubry and Ken Clarke.

Supporters of the amendment, however, indicated it may not be moved to a vote on Monday night, partly because Remain-supporting Conservatives do not want to undermine the prime minister when she is vulnerable on her right flank. It could be debated by MPs when the bill returns from the Lords, they added.

The bill is also due to have its third reading tomorrow, and it could be in jeopardy if the angry Tory Brexiters join with Labour, the SNP and the Lib Dems, who are expected to vote against it.

Chuka Umunna, a pro-Remain backbench Labour MP, said that the lack of certainty over the bill demonstrated how unclear the UK’s future was: “There is no majority in parliament for the prime minister’s Brexit, a Jacob Rees-Mogg hard Brexit or a no-deal Brexit favoured by many of the ERG. But there is a Commons majority for the UK to participate in the customs union, if some Tory MPs put country before party.”

May is due to visit the Farnborough air show on Monday morning in an attempt to highlight that her Chequers plan can support manufacturing jobs, a month after aircraft manufacturer Airbus warned that a bad Brexit deal could lead to it relocating jobs to mainland Europe. She will open the event, announcing £343m of R&D research and development funding for the sector and say her Brexit plans will support “growth, maintaining security and safeguarding British jobs”.

Key political events next week

MondayTheresa May expected to update MPs on last week’s Nato summit. The taxation (cross border trade) bill returns to the Commons with the Tory right planning a show of strength in support of hard Brexit amendments, while opposition parties could press a vote demanding that May negotiate staying in the customs union.

TuesdayTrade bill comes to the Commons for its final stages. Nine Tory Remainers have signed amendments calling for the government to stay in a customs union with the European Union after the UK quits in 2019.

WednesdayInformal deadline to hold a no-confidence vote in Theresa May. If the number of Tories prepared to call for one has not reached 48 by today, then there is no longer enough time to hold a confidence vote before parliament breaks up for the summer.

Theresa May also faces Jeremy Corbyn for the last prime minister’s questions for the last time before summer recess.

FridayMichel Barnier meets with EU ministers in Brussels to discuss Britain’s fresh Brexit proposals, the first time the 27 member countries have formally discussed May’s political initiative.